Tubes cause at least 50% of tube amp
problems. If you have a tube amp and you haven't tried changing all the
tubes, then do that first. Even problems that you wouldn't associate with
tubes could be the tubes. If changing all the tubes fixed the problem,
then go back and try to figure out which tube it was and replace that one.
If it was a power tube (usually the bigger ones, and there are usually
an even number of them), then you probably have to have the amp biased.
It's always best to change all the power tubes at once, not replace one
at a time.

When gigging with a tube
amp, always bring at the very least one of every tube in the amp and some
proper sized fuses for the amp. It's even better to just bring a
backup amp. We that use tube amps must take the bad with the good. We pay
a higher price for the amps and maintenance, and to a lesser extent pay
for expected breakdowns all for the payoff of awesome tone (jeez do I sound
like a Finance major yet?)! You have to take the bad with the good and
assume that your amp is going to break sometime. They all do.

These tips are for blackface/silverface
Fenders. But the logic in some of these tips can be used on other amps
as well.

I highly
recommend getting Gerald Weber's books. There is quite a lot of self promotion
in there, and a few inaccuracies, but they are good. This guy knows his
stuff and is very generous with his knowledge in these books (but he does
get what, $25 each for them?). I'm not going to go through biasing on the
web page, but Gerald does in the books. Get the first one first (hip tube
amps). I've found that a lot of the things Gerald has found in fixing problems
mirrors my experience exactly!

Remember,
tube amps have voltages that will shock the daylights out of you, even
when unplugged. Don't go inside it if you don't know how to drain filter
caps!

Troubleshooting at the gig
(or troubleshooting without opening up the amp)

Troubleshooting at the gig assumes you
have no time to open up the amp and look inside or have much time to do
anything at all! It also assumes that if these suggestions don't fix the
amp, that you need more time and probably need it opened up. This is about
the extent of troubleshooting you can do without opening the amp.

1. The amp blows fusesThis is a common problem. Usually it's
a shorted rectifier or power tube. Take all the tubes out of the amp, replace
the fuse and turn on both switches. If the fuse blows then you probably
have a shorted filter cap or in extreme circumstances, a blown power transformer.
This is not going to be fixable at the gig. If the light stays on when
you take all the tubes out, shut the amp down and install the rectifier
tube (if it has one), then the power tubes, then the preamp tubes one by
one turning the amp on every time you install a new tube. When the fuse
blows, you've found the bad tube. Sometimes the fuse will blow if you simply
flip both switches on at the same time (which you should never do! Shame
on you if you do this! Let the tubes warm up about 30 seconds before you
turn on the second switch!). If you find that the fuse blows after replacing
one of the power tubes, and still blows when you install a known good power
tube, you could possibly have a blown output transformer.

2. There is a crackling sound
that happens all the time or sporadicallyThis is usually bad preamp tubes. Unplug
tubes one at a time going from right to left looking from the back while
the amp is making noise. The one that stops the noise is the one you want
to replace. Replace it with a known good one and see what happens. If you
still have noise, you very likely have a bad component somewhere inside
the amp.

3. Controls on amp are very
scratchy and make too much noise to adjust on stageTurn off the amp and turn the noisy controls
vigorously through their whole range. This clears the garbage out of their
path temporarily. This will probably get you through the gig. You should
have the pots cleaned with pot cleaner though. Or possibly have the pot
replaced. If this doesn't fix the pot, then you may have other problems.

4. Reverb doesn't workMake sure the reverb send and return cables
are plugged in tight, and the red one is on the right. Replace the third
tube from the right looking from the back (second on a Princeton Reverb).
This is the reverb driver tube (12AT7). Replace the fourth tube from the
right looking from the back (third on a Princeton Reverb). This is the
reverb recovery tube (12AX7). If all this doesn't fix it, try plugging
into a different reverb pan from that backup amp you should have brought!

5. Tremolo doesn't workReplace the fifth tube from the right
(forth on a Princeton, second on a Vibro-Champ, third on a non reverb amp)
looking from the back (12AX7). If that doesn't do it you should check to
make sure that your footswitch is working properly. The tremolo will not
work on blackface/silverface Fenders without the footswitches plugged in
on almost all the amps.

6. One channel works but
the other doesn't (not applicable to Princeton, Champ)If the vibrato channel works but the normal
one doesn't, replace the first tube on the right looking from the back
(12AX7). This is the first and second gain stages for the normal channel.
This is the only tube associated solely with the normal channel. If the
normal channel works but the reverb channel doesn't work, replace the second
and fourth tube from the right looking from the back (12AX7's).

7. Both channels don't work
but there is still hiss coming out of the speakerChange the sixth tube from the right looking
from the back (12AT7). This is the phase inverter tube and is the only
one associated with signal on both channels. It is also the tube that is
nearest to the power tubes on most amps.

8. Amp makes sound but is
very low level and distorts easilyMake sure the speaker is plugged into
the right jack. Plugging into the ext. speaker jack will make the amp very
quiet and it will distort at a low level. This is bad for the amp. It's
a good way to damage it in short order. Also, make sure all the tubes are
plugged in securely. If this still doesn't do it, start replacing tubes
starting at the right and going left (looking from the back).

Troubleshooting
at home

Once again, don't
do this unless you know how to discharge filter caps! You can shock the
daylights out of yourself even if the amp is unplugged.

First I will go
through some general maintenance type tips. All of these require removal
of the chassis. You'll need a soldering iron and meter too.

1.
Clean the potsGet some electrical
parts cleaner that is made for cleaning pots. It will have a lubricant
in it which is what pots need. A lot of people recommend Caig Deoxit, but
I just use some stuff I get at the local warehouse. Spray a little cleaner
in from the top where the terminals are on the pot. Turn it through it's
range a few times to get the cleaner everywhere. If the pot still
makes noise after cleaning and turning it a few times, you could have problems
elsewhere in the amp.

2.
Tighten the potsMany of the pots
need to have good contact with the chassis because their ground is part
of the circuit. Tweeds especially. Check the resistance from the body of
the pot to the chassis. If you get anything more than an ohm or two, then
you need to either tighten the pot or take it out and turn the star washer
underneath. I've found pots that have a resistance from the body to ground
of higher than 1k! This will cause problems if something is grounded to
the back of the pot! Spraying the chassis where the pot makes contact would
be a good idea too.

3.
Check for bad solder jointsThese things show
up on Fenders all the time and cause some very strange problems. The circuit
board usually isn't the problem. All the components were bent and run completely
through the eyelets and got a good joint usually. The ones to look for
are the grounds going to the brass grounding strip under the pots. These
will frequently lift up a little. Or they may not even look bad till you
look really close. You'll need a fairly hot iron to reheat these. It's
not a bad idea to remove the old solder and put in new. Another place to
look for bad joints is the tube sockets. These are sometimes hard to find.
The preamp tubes are especially suspect. If you see what looks like a loose
wire or even a little bit of a gap where the wire goes through, remove
the old solder and put in some new. The pots with components grounded to
the back of the pot are worth looking at too.

4. Check for broken component
leadsThis happens quite often. If you poke
around in the amp with a non conductive probe (like a plastic pen or marker),
you'll sometimes find that the lead has broken off a component. It happens
right where the lead goes into the body of the component. This happens
to resistors, and to Mallory filter caps most often. If you look right
up near the body of the component, it sometimes looks like the lead is
misaligned. If you poke it, the lead may move. Those old carbon comp resistors
can crack right through the middle too. Look underneath at the filter caps
as well, these are not immune. But these should probably be changed anyway!

5.
Remove solder in places where there is too muchSome people seem
to think that globbing a bunch of solder on a joint is the way to keep
it together. This is not true. I spend a lot of time sucking big globs
of solder out of amps that someone worked on. Just add enough solder that
it covers the joint and holds securely. Solder is for making a connection,
not physically holding things together.

6.
Fix things that look obviously hackedSome people have
no respect. I see amps with wires all over hell duct taped together and
wire nutted. Put in clean and straight wires here. No wire nuts or other
crimp type devices inside the amp! It's usually much easier just to replace
the whole wire than try to splice it with something that could eventually
fail anyway. A lot of times you'll see components soldered on to what looks
like the leads of an older component that was cut out. If components look
like they have their leads soldered to old leads that were cut, fix it.
Just get a new cap or resistor and take out all the old garbage. Your amp
will thank you! Besides, doesn't it feel good to get the amp back in shape?

7.
Tighten everything downTransformers come
loose, as do jacks and everything else. Make sure everything is tight,
but don't crank on things too hard. We are dealing with self threading
sheet metal screws, they can't be tightened too hard. Speakers always come
loose after time. You should check all this stuff periodically too.

8.
Change the filter capsJust do it. Even
if they are not bad or causing problems, just change them. If the amp is
anywhere near about 20 years old or older, the caps need changing. Don't
forget the cathode bypass caps and the bias supply filter cap!

9.
Retension the tube socketsIt is important
that you drain filter caps before you do this. Don't try to retension sockets
with the chassis still in the amp. The plate pin of the power tube will
shock the crap out of you. Don't do it!This really only
works on the power tube sockets. If you look down in there you'll see that
there is a curved part of the pin and a flat part (not always, some of
the newer amps have different kinds of sockets). Take a little tiny screwdriver
or a paper clip and bend the flat part inward so they contact the pin better.
Don't bend these too much, because if you do, the pin of the tube will
come in and get caught on the pin that you bent in too much. It can sometimes
bend the pin of the socket down and your tube won't go into the socket.
This pretty much ruins the socket. Be careful! The preamp tube sockets
aren't really easily worked on. This is something that I do if I suspect
that a preamp tube isn't making good contact. First, try putting the tube
in a pin straightener if you've got one. That may do the trick. But what
I do is bend the pins of the preamp tube inward VERY SLIGHTLY. I want to
emphasize VERY. If you go at it with a hamfist, you will crack and ruin
the tube. As a matter of fact, the tube may be weak anyway and it will
crack even if you are careful. Make sure you bend all the pins in about
the same amount. If you bend them too much, then the pins of the tube can
get hung up on the socket and you'll bend them even more and possibly break
something. If you've done this correctly, you'll notice that the tube goes
in a little tighter than before, which should make better contact than
before. If you can't do this or it doesn't seem to fix anything,
just get a new socket. They are cheap.

10. Check for leaking coupling/blocking
capsIn older amps, especially Fender tweeds
in my experience, the blocking/coupling caps can start leaking DC. The
job of these caps is to keep high DC voltage from getting to places where
it will cause damage or make the circuit not work (grids of tubes or pots
come to mind). Usually one side of a blocking cap will have DC on it, and
the other will have none. This is not always the case though, so be sure
to check the schematic (you check these by lifting the side that has the
lower voltage on it). If you find much more than .1 volts DC on the side
that there isn't supposed to be DC, then the cap is a candidate for changing.
In tweed amps, the red or yellow Astrons are almost always leaky. In blackface,
there are some reddish brown ceramic caps that are candidates for changing.
The ones in the tremolo circuit, and the .02 in the vibrato channel are
bad more often than not. Also, the 500pf and 250pf's are worth looking
at.

Amp hums very loud

This could be many things, and there are
many different kinds of hum! I'll go through a few things here.

1. Amp buzzes loud when you
take your hands off the stringsThis is going to sound incredibly simplistic,
but it happens! Usually, switching the ground switch on the back of the
amp fixes this. Not many people are aware of this, and I get asked this
question more than you'd think ("Why is my amp buzzing?"). If your amp
doesn't have a ground switch and has a 2 prong cord, take the plug out
of the wall and turn it around. This changes the ground refrence in the
chassis which is the same effect as the ground switch.

2. Silverface amp/blackface
Bassman hums loudlySilverface amps, and the blackface Bassman,
had a bias balance circuit that would make the amp hum very loud if it
wasn't set right. You can turn the bias pot a LITTLE either way and see
if it reduces hum. If no difference is heard, stop turning it and put it
back to about where it was. These should be converted to the standard blackface
bias circuit anyway. Some later silverface amps had a "hum balance" control
too. This is a pot on the filament supply that balances it. You can turn
that one with no damage. It's best to adjust that pot with the amp turned
up loud but with nothing connected to the input. Adjust it untill you hear
the least amount of hum.

3. Change the tubesThe filament in a tube can cause hum.
When tubes get old, sometimes they hum. Try unplugging tubes going from
right to left looking from the back. When you pull out a tube that reduces
or eliminates hum, try a new one in that position. The rectifier tube could
be causing hum as well. Sometimes in a 4 power tube amp, one power tube
will fail and this will cause the amp to hum.

4. Change the filter capsI know I know, I'm sounding like a broken
record here. But when filter caps lose their filtering capability, they
cause the amp to hum. Their whole job is to turn AC into DC the best they
can, which is partly to reduce hum! The bias supply cap is also suspect.

5. Check for bad grounds
and bad solder jointsOne more thing to check that I've already
mentioned. Make sure that all solder joints look solid. Poke around inside
the amp with a pencil or piece of wood to make sure that solder connections
are good.

6. Check your input jacksThe input jacks on amps are usually shorted.
This means that when you unplug your guitar cord, the jack gets shorted
to ground. If these shorts don't work, the amp may not work right (in the
case of a JCM800 Marshall, if the short isn't working on your low gain
input, your high gain input won't work!), or cause it to hum. These can't
be retensioned. Just get new ones!